Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in the hot sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBCs) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, TBCs must have low thermal conductivity, be capable of strongly adhering to the article, and remain adherent through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between low thermal conductivity materials used to form TBCs, typically ceramic, and materials used to form turbine engine components, typically superalloys. For this reason, ceramic TBCs are typically deposited on a metallic bond coat that is formulated to promote the adhesion of the ceramic layer to the component while also inhibiting oxidation of the underlying superalloy. Together, the ceramic layer and metallic bond coat form what is termed a thermal barrier coating system. Typical bond coat materials are diffusion aluminides and oxidation-resistant alloys such as MCrAlY, where M is iron, cobalt and/or nickel.
Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray or vapor deposition techniques. A continuing challenge of thermal barrier coating systems has been the formation of a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling. In one form, improved spallation resistance is achieved with ceramic coatings deposited by electron beam physical vapor deposition (EBPVD) to yield a columnar grain structure. Such grain structures are characterized by gaps between grains that are oriented perpendicular to the substrate surface, and therefore promote strain tolerance by enabling the ceramic layer to expand with its underlying substrate without causing damaging stresses that lead to spallation.
FIG. 1 represents a coating apparatus 20 for depositing ceramic coatings by EBPVD in accordance with the prior art. The apparatus 20 includes a coating chamber 22 in which a component 30 is suspended for coating. A ceramic layer 32 is deposited on the component 30 by melting a ceramic ingot 10 with an electron beam 26 produced by an electron beam (EB) gun 28. The intensity of the beam 26 is sufficient to produce a stream of ceramic vapor 34 that condenses on the component 30 to form the ceramic layer 32. As shown, the ceramic vapor 34 evaporates from a pool 14 of molten ceramic contained within a reservoir 18 formed by a crucible 12 that surrounds the upper end of the ingot 10. Crucibles of the type shown are often made of copper, though it is foreseeable that other materials could be used. Cooling passages 16 maintain the crucible 12 at an acceptable temperature. Because the crucible 12 must closely fit around the ingot 10 to prevent leakage, the size of the pool 14 is determined by the size of the ceramic ingot 10, which has a typical diameter of about 6.3 centimeters. As it is gradually consumed by the deposition process, the ingot 10 is incrementally fed into the chamber 22 through an airlock 24. During deposition, the chamber 22 is typically maintained at a pressure of about 0.005 mbar.
Zirconia-based thermal barrier coatings, and particularly yttria-stabilized zirconia (YSZ) coatings, produced by EBPVD to have columnar grain structures are widely employed in the art for their desirable thermal and adhesion characteristics. Nonetheless, there is an ongoing effort to improve deposition processes for thermal barrier coatings, particularly in terms of improved deposition efficiency and spallation resistance.